Compression refrigeration, chilling, heat pump, and related apparatus typically employing chlorofluorocarbon (CFC), hydroflourocarbon (HFC), replacement or alternative refrigerants are known in the art. (All kinds of such apparatus will be referred to generally as a "refrigerator" here, for brevity.) The primary components of a compression refrigerator are an expansion valve, an evaporator, a compressor and a condenser, connected in that order to form a closed refrigerant loop.
In a compression refrigerator, the refrigerant may be an azeotrope. This means that the refrigerant cannot be distilled into separate components having different compositions when it is evaporated or condensed. The composition of the refrigerant in its liquid or vapor forms is identical.
The evaporator of a compression refrigerator is a specialized heat exchanger. In operation, a liquid refrigerant is distributed via an expansion valve into the evaporator. A fluid to be cooled is separately introduced into the evaporator. The fluid to be cooled carries the heat load which the refrigerator is designed to cool. The evaporator transfers heat from the heat load to the liquid refrigerant.
For example, the fluid to be cooled in the evaporator may flow through the evaporator within the runs of a bundle of pipe having a heat-conductive wall, and the liquid refrigerant entering the evaporator may be distributed on the outside of the pipe bundle. The outside of the pipe bundle can be referred to as a heat exchange surface. The conditions in the evaporator are arranged so heat is transferred from the fluid to be cooled to the refrigerant through the heat exchange surface. This heat transfer boils and/or evaporates the refrigerant, forming a refrigerant vapor.
The refrigerant vapor is exhausted from the evaporator by the pumping action of the compressor. The compressor also compresses the refrigerant, forming a more dense vapor. The compression process heats the vapor, thus preventing it from condensing at this point. The compressed vapor is then transported to the condenser, which is located between the high-pressure side of the expansion valve and the high-pressure side of the compressor.
The condenser is another specialized heat exchanger. The condenser transfers the heat resulting from compression of the refrigerant and heat load received from the evaporator to a heat sink, such as ambient air, ground water, or the like. As it cools down, the compressed vapor condenses to liquid form. Finally, the cooled, condensed refrigerant passes through the expansion valve whereupon the refrigerant pressure and temperature are reduced and the cycle is repeated.
One feature of a compression refrigerator is that, typically, the bundle of pipe carrying the heat load in the evaporator is immersed in a standing body of the liquid refrigerant. This type of evaporator is called a "flooded evaporator". U.S. Pat. No. 4,829,786 to Sand et al. is exemplary of this type of evaporator, is assigned to the assignee of the present invention, and is incorporated by reference herein.
Another characteristic of a compression refrigerator is that the oil for lubricating the compressor circulates with the refrigerant, and collects in the evaporator. This occurs because the oil is less volatile than the refrigerant. Thus, when the refrigerant leaves the evaporator as a vapor, the less-volatile oil is left behind. In a system employing a flooded evaporator, most of the lubricating oil is mixed in with the charge of liquid refrigerant in the evaporator. This oil is not a good refrigerant, interferes with heat transfer, and is prevented from carrying out its primary mission: to lubricate the compressor.
Another known type of refrigerator is an absorption refrigerator. An absorption refrigerator differs from a compression refrigerator in several respects. One difference is that an absorption refrigerator employs a composite or non-azeotropic refrigerant. A second difference is that an absorption refrigerator includes a generator and an absorber in the refrigerant loop.
A variety of composite or non-azeotropic refrigerant systems can be used in an absorption refrigerator. Two examples are an ammonia/water system and a lithium bromide/water system. Non-azeotropic refrigerants are intentionally distilled into two components-a more--volatile and a less-volatile component--during operation of the refrigerator. The two components are separated in the generator, follow different paths through the apparatus, and then are recombined in the absorber.
Between the generator and absorber of an absorption refrigerator, the separated, more-volatile component of the refrigerant is routed through a condenser and evaporator which function comparably to the condenser and evaporator of a compression refrigerator. Since no lubricating oil is provided in an absorption refrigerator, lubricating oil does not tend to collect in its evaporator.
In absorption apparatus, the evaporator is a falling-film evaporator having vertical or horizontal tubes that are sprayed from a horizontal direction. The fluid to be chilled is typically conveyed through the interior of a bundle of pipe. In practice, the liquid refrigerant, typically water in an absorption system, is sprayed horizontally by a sprayer so that it contacts the outside of the pipe bundle. The bundle is arranged so the refrigerant will flow down along the heat-exchange surface from the top of the pipe bundle to the bottom. Rather than optimizing the amount of liquid refrigerant, copious amounts of the refrigerant are oversprayed on the vertical tubes. The tubing bundle can include vertical runs of pipe, horizontal runs of pipe, coils of pipe running generally circumferentially about a vertical axis, other configurations, or combinations of these.
U.S. Pat. No. 4,918,944 (Takahashi et al.) is an example of one type of falling film evaporator. Other patents which may be pertinent are U.S. Pat. Nos. 3,213,935 (Reid), 3,240,265 (Weller), 3,267,693 (Richardson et al.), and 5,036,680 (Fujiwara et al.).
Compression refrigerators may also use direct expansion (DX) evaporators where the refrigerant is within a tube and the fluid being cooled is external to the tube. The oil return mechanism in a DX evaporator differs from flooded evaporators and the DX evaporators are generally used where 50 tons or less of cooling is desired since dual circuitry or derating is required for higher tonnages.
Compression refrigerators therefore have been distinguished from absorption refrigerators by the type of refrigerant used (azeotropic versus non-azeotropic), by the mechanism used to return the refrigerant to its initial condition (i.e. mechanical compressor versus generator), by the type of evaporator employed (flooded only for a compression system versus either type for absorption), by the tendency only of compression refrigerators to undesirably collect lubricating oil in the evaporator, and in other ways.
Flooded evaporators have a number of disadvantages distinct to their design. They use more refrigerant and more lubricant than falling film systems, thereby increasing system cost. Also, the liquid refrigerant at the bottom of the evaporator vessel will only boil and evaporate at a relatively high temperature because of the hydrostatic head or pressure from the liquid refrigerant in the evaporator vessel. Because the liquid refrigerant near the bottom of the evaporator vessel will only boil at a relatively high temperature, less evaporation occurs and less heat is removed by the refrigerant. This makes the refrigerator less efficient. Further, the lubricating oil trapped in the flooded evaporator of a compression refrigerator is difficult to separate because the charge of refrigerant is turbulent in the evaporator. Turbulence tends to continuously mix the lubricant and refrigerant, interfering with their separation.
Current falling film evaporators have their own disadvantages. Many falling film evaporators deposit an excess of refrigerant on the top of the bundle to ensure complete wetting of the heat exchange surface from top to bottom. The excess liquid refrigerant that inevitably reaches the bottom of the evaporator vessel is collected in a sump, then recycled from the sump to the top of the tube bundle. Such recycling, falling film systems have been unsuitable for use with compression refrigerators because the recycling of the refrigerant would leave a relatively high proportioned mixture of entrained lubricant on the lower parts of the tube bundle and in the sump. This mixture would contain a high concentration of lubricant since most of the refrigerant in the evaporator vessel has already evaporated before reaching the bottom of the evaporator if the system is working properly. Unfortunately, recycling the lubricant-rich mixture over the heat exchange surface in such systems decreases system efficiency because the lubricant distributed heavily over the heat exchange surface reduces the ability of the system to evaporate the refrigerant. Thus, falling film evaporators, and particularly recycling falling film evaporators, have not been used in compression refrigeration systems.
Another problem is common to falling film evaporators: the precise control of liquid refrigerant distribution. Most falling film evaporators spray the liquid refrigerant onto the heat exchange surface (typically a series of tubes carrying liquid to be cooled). The sprayed liquid refrigerant tends to splash off the surface, thereby reducing the intimacy of contact between the refrigerant and the heat exchange surface. Because the refrigerant is in less-intimate contact with the heat exchange surface, it will have less chance to boil and remove heat from the liquid inside the tubes comprising the heat exchange surface. Additionally, if the heat exchange surface is a stack of horizontally disposed tubes, it is difficult to control the axial distribution of liquid refrigerant along the length of the tubes. Therefore some parts of the heat exchange surface may be cooled while others are not. Finally, droplets of the sprayed refrigerant can form a mist or aerosol in the refrigerant vapor that can be sucked into and damage the compressor of the refrigeration system.
In U.S. Pat. No. 5,036,680 to Fujiwara et al., the pressurized refrigerant leaving the expansion valve is separated into liquid and vapor phases in a vapor-liquid separator outside the evaporator vessel. The liquid refrigerant is then transported via a pipe to the evaporation vessel and distributed on the heat exchange surface, while the refrigerant vapor is separately conveyed to the exit of the evaporator.
A system having an external separator must transport the liquid refrigerant over a distance through a pipe to carry it into the evaporator vessel. Because the liquid refrigerant is near its boiling point, it is difficult to transport evenly through a pipe, for two reasons. First, the pipe has a relatively high surface area in contact with ambient air. The pipe is therefore capable of receiving ambient heat and evaporating the liquid refrigerant, forming bubbles or foam in the liquid refrigerant which can prevent the liquid refrigerant from flowing evenly within the pipe. Second, the pipe from the separator to the evaporator typically has not been a straight run. Elbows, joints, and the like in the pipe form areas where bubbles or foam can collect and interfere with efficient refrigerant distribution even more.
Also, the vapor-liquid separator takes up space outside of the evaporation vessel, increasing the total system size. Finally, systems utilizing a separate vapor-liquid separator are usually more costly because the separator vessel, like the evaporator vessel, must be built and certified to withstand high pressure.